Storage tube



R. S. MOORE April 8, 1958 STORAGE TUBE Filed April 24. 1953 \oov T xww ua N p INVENTOR- HDBERT $.MEIDRE WQY United States Patent STORAGE TUBE Robert S. Moore, Syracuse, N. Y., assignor, by mesneassignments, to the United States of America as represented by the Secretary of the Navy Application April 24, 1953, Serial No. 350,905 Claims. 01. 315-12 This invention is directed to an electron discharge device and more particularly to a storage tube for the conversion of one type of electrical signal to another type of electrical signal.

The invention relates specifically to a storage tube having an insulating target upon which a signal charge pattern isestablished. The charge pattern is used to provide output signals of the tube corresponding to the charge pattern. The output signals areutilized to provide a visual picture of the pattern. One storage tube of this type is that which is fully disclosed in U. S. application Serial Number 29,746 of L. Pensak, filed May 28, 1948. One modification of this type of storage tube utilizes a target electrode having a thin film of insulating material thereon. An electron beam is directed against one side of the target film to provide a charge pattern on the insulating film; Asecond electron beam is scanned over the charged surface of the target film to discharge the film and provide an output signal from the tube, Which may be amplified and fed into a kinescope or viewing tube to provide a visual picture of the charge pattern on the storage tube target.

Such a storage tube consists essentially of a first electron gun, called a writing gun, directed atone side of the target electrode and a second electron gun, known as the reading gun, directed at the opposite side of the target. Both of the guns are substantially on the same axis and a the target electrode is positioned intermediate the guns and substantially normal thereto. The target is formed with a supporting mesh screen coated on oneside with an electron pervious aluminum film. A thin film of insulating material is supported on the exposed surface of the aluminum film.

the target surface to become charged to an equilibrium potential which is positive with respect to the aluminum film.. There is an output from the tube even though no r 2,830,225 Patented Apr. 8, 1958 target surface, charges the surface from point to point back to the equilibrium potential and by an amount depending on the potential at each point set by the writing gun. This charging of the target surface provides successive signal pulses in the target circuit, which can be amplified and fed to the kinescope or viewing tube so that the charge pattern on the storage tube is visually represented. The collection of secondary electrons from portions of the target which are not discharged by the writing beam, provide the background signal or black level of the picture on the kinescope. This collection of secondaries from these areas should be uniform to provide .a uniform black level of the kinescope picture.

However, the signal output from such a storage tube has a non-uniform black level or background which causes a shading or decrease in contrast of the picture. The background shading a signal is higher in the center of the picture than at the edge. This higher background signal in the central part of the target results from the fact that more secondary electrons are collected from portions of the target near the centerwhich are not discharged by the writing beam than are collected from target portions near the edge which are not discharged by the writing beam. The metallic ring for supporting the target is normally operated at about 40 volts negative, and it has been determined that this negative potential sets up a retarding field between the target and the conductive coating or collector near the outer edges of the target so that secondaries generated near the edge of thetarget tend to be directed towards the central part of the target and collected by the positive areas there. This rain of secondaries charges the positive areas negatively below equilibrium and therefore produces a non-uniformity in the background signal. Fringe fields from the deflecting the writing gun, and is independent of shading. The

usable output signal of the tube is equal to the difference between the signal resulting from the penetration of the writing beam through the target surface and the background signal.

A reduction of the background or shading signal, therefore, results in a usable output signal having improved contrast and relatively higher amplitude.

' A principal object of this invention is to provide an improved storage tube.

signals are being written onto the target by the writing a gun. This signal is referred to as the background signal and is due to the secondary electrons driven off the target surface by the electronbeam of the reading gun. However, only as many secondaries can leave the surface as are replaced by the impinging electron beam of the reading gun. V

Thewriting gun provides a high velocity beam which, when modulated, for instance,-gpenetrates through the aluminum and insulating films and discharges the surface of the insulating film to the potential of the aluminum film at'all points penetrated. Thus, whenthe writing gun beam is modulated by a signal applied to the gun, there is established on the targetsurfacea charge-pattern corresponding to the incoming signals applied to the writing gun. Thereading gun then, upon scanning-the-charged' Another object of this invention is to provide a storage tube in which'the background signallevel is substantially uniform irrespective of the part of the target where said output signal is derived.

Still another object of this invention is to provide a storage tube in which ratio between the modulation signal amplitude and the amplitude storagesignal are more nearly constant over the entire surface of the target on i which information is stored.

The above and related objects are achieved in accordance with the present invention by the addition'of a shield ring to the target structure. The shield ring, insulated from the target and extending along the longitudinal axis of the tube in the. direction of the conductive wall coating or collector on the reading end of the tube, is operated at a positive potential to overcome thepreviously mentioned repelling fields which tend .to deflect secondary electrons towards the center of the insulating film. Thus, the rate' of collection of electrons on the conductive wall coating or collector electrode is sub stantially uniform regardless of which area of the target from whence the electrons were emitted.

The invention will best be understood by reference to the following description taken in connection with the accompanying drawing, in which: j

Figure 1 is a crosssectional view of a storage tube utilizing this invention;

Figure 2 is an end view of the target assembly of the tube shown in Figure l, and s Figure 3 is an enlarged partial sectional view of Figure 2.

Figure 1 shows an electron discharge device. comprising a storage tube having a tubular envelope portion 1.0. At one end of the envelope there is mounted an electron gun 12 to be described as the writing gun of the tube. At the opposite end of the envelope there is mounted an electron gun 14 to be known as the reading gun of the tube. Intermediate the electron guns 12 and 14 there is positioned a target electrode 16, positioned transversely to the axes of the electron guns 12 and 14. i

The storage tube 10 is a type which has been fully described in the copending application of Pensak cited above. The writing gun 12 consists principally of a cathode electrode 18 mounted within a cup like control grid electrode 20 and coaxially aligned with a first accelerating electrode 22 and the second accelerating electrode 24. Electrodes 20, 22 and 24 are mounted successively along a common axis by any well-known means such as glass support rods 26 sealed to metal studs projecting from each electrode, as shown. Cathode 18 consists primarily of a closed cylinder With the closed end facing target 16 and being coated with an electron emitting material. The electrodes 20, 22, and 24 have apertures therethrough on the common axis of the gun to provide a passage for electrons emitted from the cathode coat- Cil ing. During tube operation, the cathode emitting surface is heated to provide a copious emission of electrons which are drawn by the positive field of the accelerating electrode 22 away from the cathode surface. These electrons pass through the apertures of electrodes 20, 22 and 24, while the fields between the electrodes form the electrons into a beam, which is brought to a minimum cross sectional area or spot on the surface of target 16. The final focusing of the electron beam of gun 12 on target 16 is provided by the field established between the tubular electrode 24 and a conductive Wall coating 30 applied to the inner surface of the tube envelope between gun 12 and the target 16. The parts of the electron gun 12 are conventional and the electron optics of such a gun and its operation are well-known and are not further described herewith as they do not constitute a part of this invention.

As shown in Figure 1, the several electrodes of the gun 12 are connected to a voltage divider 28 for providing operating voltages to the respective electrodes of the gun. A set of voltages is indicated in the drawing. These voltages represent values which have been used in the successful operation of a tube of the type described, but are not meant to be limiting.

Electron gun 14, the reading gun, includes a cathode electrode 32 and a control grid electrode 34 similar respectively to electrodes 18 and 20 of the writing gun 12. Mounted successively along the common gun between the control grid 34 and the target electrode 16 are an accelerating electrode 36, a focusing electrode 38 and a second accelerating electrode 40. Accelerating electrodes 36 and 40 are connected directly to each other through a common supporting metal cup or gun holder 42. The cathode-control grid assembly of gun 14 is rigidly mounted to the accelerating electrode 36 by means of glass support rods 44, as shown. A conductive coating 46 extends over the inner surface of the envelope 10 from the accelerating electrode 40 to a point adjacent to the screen target electrode 16.

are also connected to external sources of potential for" providing operating potentials to the gun electrodes. During tube operation, the gun 14 forms an electron beam with the electron emissionfrom the cathode 32, which is directed onto the target 16 and focused to a fine spot at the point it strikes the target.

The beams emanating from guns 12 and 14 are re spectively scanned over the surface of target 16 by defleeting yokes 48 and 50. These deflection yokes are of conventional design, in that they consist of two pairs of deflecting coils mounted in the yoke with the coils of each pair on opposite sides of the tube envelope 10. The coils of each pair are connected in series to a source of saw toothed currents for providing a magnetic field in the path of the respective electron beam. The fields of each yoke 48 or 5% provide line and frame scansion of the respective electron beams over the surface of target 16. Such deflection coils and their modeof operation is well-known in the prior art and since they do not constitute a part of this invention, they are not described in further detail.

The envelope 10 of the tube is formed from several tubular sections. The portion, for example, enclosing the gun 12 is formed from a pair of glass tubular members 52 and 54 which are connected together through a metal alignment cylinder 58 sealed thereto. In a similar manner the portion of envelope 10 enclosing gun 14 is formed from a pair of tubular glass members 60 and 62 which are joined together through an intermediate alignment cylinder 64.

Target electrode 16 is mounted adjacent the junction 66 where the envelope portions 54, 60, housing the respective guns 12 and 14 are sealed together to close the'envelope. Target 16 comprises a thin electron pervious metallic film, such as an aluminum film 68, having on its surface facing gun 14 a thin film 70 of insulating material such as magnesium fluoride, for example.

The operation of the tube is substantially that in which I input signals are applied to the control grid 20 of gun 12 to modulate the electron beam of the gun while it is scanned over thesurface of target 16. The aluminum target film 68 is maintained in the order of a --40 volts negative to ground. The electron beam of gun I4 is unmodulated and is scanned over the surface of the sion. The beam of electrons from the electron gun 14 will strike the dielectric target film 70 at energies in the order of 1,000 vol-ts. These energies are between the first and second crossover potentials for the magnesium fluoride screen so that at every point on target film 70 where the beam of gun 14 strikes the ratio of the secondary electron emission current from the film to the incident electron current is greater than unity. The secondary electrons are drawn away and collected by the positive collecting coating 46. The loss of secondary electrons from each point of film 7 under the scanning beam of gun 14 raises the potential of that point positively to a level close to the potential of coating 46 which is held at ground potential. In this manner then the target surface scanned by the beam of gun 14' is raised to an equilibrium potential which is that potential level at which the secondary electrons leaving the target surface are equal in number to the primary electrons of the beam of gun 14 striking the surface.

The modulated electron beam of gun 12 strikes the target with energies in the order of 9,000 volts, which is suflicient to cause the electrons of the beam of gun 12 to penetrate through the target films 68 and 70 as described in thev above cited copending application of L. Pensak. The magnesium fluoride film 70 is amaterial which is normally insulating, but which becomes conducting when struck by high velocity electrons at those points where the electrons penetrate substantially .librium potential.

through themagnesium fluoride film. Thus, during tube operation, the modulated high velocity beam of gun 12 beam of gun12' will discharge the positive surface of film 70 toward the negative potential of the aluminum film 68 and in proportion to the strength of the beam inducing the conductivity at each point; The scanning of the high velocity modulated beam of gun 12 over the surface of target 16, in this manner, establishes on the exposed surface of film 70 a negative charge distribution or pattern corresponding to the sequential input signals applied to the control grid 20 of gun 12.

The beam of gun 14, on scanning over the charged surface of the film 70, will instantaneously charge the areas discharged by the beam of gun 12 backto the equi- When the beam of gun 14 strikes a negative area of film 70 the secondary emission from this point is instantaneously greater than from apositive area. This instantaneous charging of anyone point of film 70 results in a corresponding pulsein the circuit 56 of aluminum film or plate causing a change in the voltage across the high resistance 72 of the plate circuit. This voltage is detected and amplified in wellknown manner to provide an output signal from the tube, which is fed to a monitor kinescope or other utilization means. Thus, as the beam of gun 14 scans the charged pattern established by the beam of. gun 12 on film 70, there is provided a succession of signal pulses which vary in'amplitude as determined by the potential of the areas charged.

Bombardment of the target 16 by the writing beam produces a flow of current in the target circuit 56 which must be separated from the target current produced by the reading beam. The separation of the two :conponents of target current may be accomplished by a time-sharing method, or, as illustrated in Fig. l, by the use of radiofrequency modulation of the reading component in conjunction with atuned output amplifier.

If the beam of gun 14 is scanned over the charged surface of target film 70 in a normal television scan, the output is a television signal which will produce on the monitor kinescope a scanning visual representation of the charge pattern writtenon film 70 of the storage tube. By controlling the beam current of gun' 14, it is possible to control the storage time of the charge pattern. on film 70. Thus, if the beam current i of gun 14is sufiiciently small, the charge pattern on I target film may remain for a determinable length of time before it is entirely erased. then, the scanning rate of the beam of gun 12 may be different from that of the beam of gun 14. Furthermore,

the pattern or scanning raster established on film 70 may be written down differently than the reading raster of gun 1 4. For example, the-writing gun 12 may be operated to provide a radar raster of the charge pattern which is laid down on'thetarget 70 in several seconds, While simultaneously the pattern may be continually read off by the reading gun 14 with atelevision scan frame time of of a second.

The storage tube and its operation, as thus described, are in accordance with prior art practice.

However, the metallic target ring is normally operated at, for example, 40 volts negative potential, and the conductive coating or collector electrode is at ground potential. Thus, part of the secondary electron emission from near the edges of the insulating film which would normally be attracted to the collector electrode 46 is repelled by the negative field of the target ring 'In this manner and falls back on the positive center portions of the insulating film rather than be collected by the collector electrode 46. This non-uniform collection of electrons,

as mentioned before, causes a shading effect in the pic-- ture due to non-uniformity of the background signal. Also, the amplitude of signals, due to modulation of the writing beam and signal storage is reduced near the edge ,of the insulating film as a result of this uniform electron redistribution. r

Fringe fields from the deflecting yokes also tend to cause secondary electrons leaving. the edge of the-t get to turn towards the center of the target.

In accordance with the present invention and, as shown more clearly in Figures 2 and 3, the target electrode 16 a is provided with a shielding structure 74 which extends. 3

towards the reading gun 14.

The target electrode 16, as previously mentioned, com-w prises a thin electron pervious metallic (aluminum, for example) film 68 on a fine mesh carrier 88. The mesh 88 is secured to the mesh support memberor ring 76 by the target retaining mesh welding ring member 78. The

film of insulating material is deposited on the surface of the metallic film 68 which faces the reading gun 14. The mesh support ring 76 is separated from the target cup 80 by the cup ring 82. The target cup 80 is insulatingly sealed to the target holder 84 by means of a fusion type seal in which a layer of glass frit 85 is sandwiched between the target cup 80 and target holder 84, insulating target 16 from target holder 84. The target 16 isretained within target holder 84 by' the shielding 1 structure or assembly ring 74 which is insulated from the mesh welding ring by an insulating ring 90, made of tantalum oxide, for example. After the target'is assembled within the target holder, assembly ring 74 is inserted and conductively secured, as indicated at 92, for

example, to the target holder 84. Target holder "84 issealed to the envelope adjacent to the junction 66 by' glass bead seals 94 between the target holder and the envelope wall. In actual practice the target holder-target cup assembly is made first, and then is sealed to the envelope wall by the bead seals 94. The cup ring 82, target 16, insulating ring 90, and shielding structure 74 are assembled after the target holder 84 has been sealed within the tube envelope in order to avoid damaging the delicate target structure 16 due to the high temperatures required to seal the target holder to the tube envelope.

External electrical contact is made to the target 16 by means of a wiper contact 96 which extends from the target cup 80 through'an aperture 98 in the target holder 84. Wiper contact 96 makes contact with the conductive pin 100 which extends through the envelope wall to become electrically connected (by means not shown) to output. circuit 56. ,External electrical contact to the shielding structure74, which is conductively connected to the target holder 84 is made by means of a pin 102 and wiper contact 104 which is conductively secured to the target holder. Pin 102 is conductively connected to a circuit 86.

The mesh welding ring member 78 as well as the metallic film 68 of the target 16 is at negative potential while the conductive coating 46 is at ground potential. Thus the field between welding ring member 78 and conductive coating 46 is such that many of the secondary electrons emitted from the edge of' the insulating target surface 70 which would normally be attracted to the conductive coating or collector electrode 46 fall back towards the center of the target. The result is the non uniform background signal which has been discussed previously.

1n accordance'with the present invention a potential is applied to the shielding structure 74, which, when properly adjusted, overcomes the efiect of the deflecting fields due both to the negative potential on mesh welding ring member 78 and the deflecting field of yoke 50.

DOB-

1? The result is that the background signal is substantially uniform over the entire target.

In a typical storage tube of the type described it was found that a positive potential of about 40 volts on the shield 74 produced excellent results when radio frequency modulation of the reading beam was used. A positive potential of about 5 volts on the shield 74 proved sulficient to provide uniform output when the reading beam was un-modulated.

Since the addition of the shield ring 74 adds to the capacitance of the target assembly, it is desirable if radio frequency signal separation is used to isolate the shield ring 74 from associated wiring capacitance. A resistor 92 of about one half megohm value has been found to be satisfactory for use as the isolating impedance.

Incorporation of the present invention in storage tubes of the type described has resulted in a shading-free target and in a 2-to-1 improvement in the signal-.to-noise ratio.

While certain specific embodiments have been illustrated and described, it will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What is claimed is: 1. A target assembly for a storage tube, said assembly comprising a target structure and a shield structure carried on a common support member, said target structure comprising an annular target supporting member, and a target stretched across said target supporting member, said target including at least a conductive layer and an insulating coating supported by said conductive layer,

said shield structure comprising a conductive member having a single unobstructed aperture, said conductive member being coaxial with said target, said shield structure being positioned adjacent to but insulated from said insulating coating of said target, the periphery of said 3 aperture of said shield structure substantially encircling the inner periphery of said target supporting member.

2. An electron discharge device for the storage of electrical signals, comprising an envelope, a target assembly within said envelope, and means for scanning ture, said conductive member being coaxial with said target, said shield structure being positioned adjacent to but insulated from said insulating coating of said target, the periphery of said aperture of said shield structure substantially encircling the inner periphery of said target supporting member.

3. An electron discharge device for the storage of electrical signals, comprising an envelope, a target as sembly within said envelope, and means for scanning each side of said target assembly with an electron beam, said target assembly comprising a target structure and a shield structure carried on a common support member, said target structure comprising an annular target supporting member and a target stretched across said target supporting member, said target including at least a conductive layer, and an insulating coating supported by said conductive layer, said shield structure comprising a con ductive member having a single unobstructed aperture, said conductive member being coaxial with said target, said shield structure being positioned adjacent to but insulated from said insulating coating of said target, the periphery of said aperture of said shield structure substantially encircling the inner periphery of said target supporting member, and separate lead means for applying an electrical potential to said tar-get structure and to said shield structure.

4. A target assembly for a storage tube comprising a cup-shaped target holder, said target holder having a centrally located substantially circular aperture in the closed end thereof, a target cup of smaller diameter than said target holder telescoped within said target holder, said target cup having a substantially circular aperture in the closed end thereof, the apertures of said target holder and of said target cup having substantially equal diameter, said target cup being insulatingly secured to said target holder, an annular target support ring within and coaxial with said target cup, a coated target comprising at least a conductive layer having insulating coating thereon stretched across said target support ring, a washer like insulator positioned across the rim of said target cup, a conductive cylindrical member of slightly smaller diameter than said target holder, said cylindrical member having an inwardly extending flange, said cylindrical member being telescoped within and said inwardly extending flange butting against said washer-like insulator, lead means for applying an electrical potential to said target support ring, and lead means for applying an electrical potential to said cylindrical member.

5. An electron discharge device for the storage of electrical signals comprising an envelope, a target assembly within said envelope and means for scanning each side of said target assembly with an electron beam, a target assembly comprising a cup-shaped target holder, said target holder having a centrally located substantially circular aperture in the closed end thereof, a target cup of smaller diameter than said target holder telescoped within said target holder, said target having a substantially circular aperture in the closed end thereof, the aperture of said target holder and of said target cup having substantially equal diameter, said target cup being insulatingly secured to said target holder, an annular target support ring within and coaxial with said target cup, a coated target comprising at least a conductive layer having insulating coating thereon stretched across said target support ring, a washer like insulator positioned across the rim of said target cup, a conductive cylindrical member of slightly smaller diameter than said target holder, said cylindrical member having an inwardly extending flange, said cylindrical extending flange butting against said washer-like insulator, lead means for applying an electrical potential to said target support ring, and lead means for applying an electrical potential to said cylindrical member.

References Cited in the file of this patent UNITED STATES PATENTS 2,549,072 Epstein Apr. 17, 1951 2,602,921 Peters et a1 July 8, 1952 2,660,669 West Nov. 24, 1953 2,667,596 Szegho et a1 Jan. 26, 1954 

